1. Field of the Invention
The present invention relates to a process for the preparation of a discoloration-free, high-molecular-weight polycarbonate which comprises melt-polycondensing a carbonic diester containing specific impurities in reduced amounts with a dihydric phenol through transesterification. Particularly, the present invention relates to a process for the preparation of a discoloration-free, high-molecular-weight polycarbonate which comprises melt-polycondensing a carbonic diester containing specific impurities in reduced amounts with a dihydric phenol through transesterification in the presence of (a) a basic nitrogenous compound and/or (b) an alkali metal compound and/or an alkaline earth metal compound as a catalyst(s).
Further, the present invention relates to a process for the preparation of a thermally stable polycarbonate. Particularly, the present invention relates to a process for the preparation of a thermally stable polycarbonate by melt-polycondensing a carbonic diester containing specific impurities in reduced amounts with a dihydric phenol through transesterification in the presence of a boric salt as a catalyst and an acidic substance which can neutralize the catalyst, or in the presence of a compound selected from the group consisting of electron-donative amines and salts thereof as a catalyst and an acidic substance which can neutralize the catalyst.
2. Description of the Related Art
Although a discoloration-free, high-molecular-weight polycarbonate is a general-purpose engineering thermoplastic which is useful in various fields, particularly as injection molding material or sheet material substituting for window pane, the processes for the preparation thereof according to the prior art had various problems.
The interfacial polycondensation process is generally effective in preparing a polycarbonate, but has disadvantages that the use of toxic phosgene is necessitated and that the formed polycarbonate is contaminated with residual chloride ion.
In order to overcome these disadvantages, Japanese Patent Publication-A No. 182336/1988 discloses a process for the preparation of a polycarbonate which comprises using liquid trichloromethyl chloroformate, which is a dimer of phosgene, instead of the toxic phosgene and polycondensing it with a special dihydric phenol by the interfacial process.
However, this patent document does not give any specific information about the special dihydric phenol with the exception of 9,9-bis(4-hydroxyphenyl)fluorenes. Further, although Angew. Chem. 99, 922(1987) describes that a polycarbonate is prepared from 2,2-bis(4-hydroxyphenyl)propane by using triphosgene instead of the toxic phosgene, a reaction mechanism wherein phosgene is generated is also described therein.
Further, it is proposed in Japanese Patent Publication-A No. 100824/1992 to use a diaryl carbonate having a xanthone content of 10 ppm or below. Although the use of such a carbonate can give a relatively light-colored polycarbonate, it has a problem that the obtained polycarbonate somewhat yellows as compared with that prepared by the phosgene process.
Furthermore, a process for the preparation of a polycarbonate using a catalyst comprising a combination of a nitrogenous basic compound with a boron compound is proposed, for example, in U.S. Pat. No. 4,590,257 (date of patent: May 20, 1986, assignee: General Electric). Although this process can give a relatively light-colored polycarbonate in spite of the poor activity of the catalyst on the polymerization, the process has problems that it takes a long time to complete the polymerization because of the low activity of the catalyst on the polymerization to result in low industrial productivity and that not only is a side reaction liable to occur during the polymerization to form a branched structure but also the obtained polycarbonate is poor in heat stability.